[Bonmin] unexpected unfeasible problem

[Nicolas BERGER]

Hi


I am trying to solve this problem :

------------------------------------------------------------------
var x0 integer >= 1 <= 10 ;
var y0 integer >= 1 <= 10 ;
var x1 >= 1 <= 10 ;
var y1 >= 1 <= 10 ;
var x2 >= 1 <= 10 ;
var y2 >= 1 <= 10 ;

maximize dist:   (x0-x1)**2 + (y0-y1)**2 + (x0-x2)**2 +
                 (y0-y2)**2 + (x1-x2)**2 + (y1-y2)**2 ;


subject to
e01: (x0-x1)**2 + (y0-y1)**2 >= 12.5 ;
e10: (x0-x1)**2 + (y0-y1)**2 <= 64 ;
e02: (x0-x2)**2 + (y0-y2)**2 >= 12.5 ;
e20: (x0-x2)**2 + (y0-y2)**2 <= 64 ;
e12: (x1-x2)**2 + (y1-y2)**2 >= 12.5 ;
e21: (x1-x2)**2 + (y1-y2)**2 <= 64 ;

solve ;
display x0,y0,x1, y1 ,x2, y2  ;
------------------------------------------------------------------------



Here is the solution to this small problem (given by gams/baron) :

-----------------------------------------------------------------------------------
                            LOWER          LEVEL          UPPER        
MARGINAL

---- VAR objvar            -INF          192.0000        +INF             .
---- VAR x0                 1.0000         1.0000        10.0000         EPS
---- VAR y0                 1.0000         6.0000        10.0000         EPS
---- VAR x1                 1.0000         7.9282        10.0000          .
---- VAR y1                 1.0000        10.0000        10.0000          .
---- VAR x2                 1.0000         7.9282        10.0000          .
---- VAR y2                 1.0000         2.0000        10.0000          .
-----------------------------------------------------------------------------------



Now here is what ampl returns when I call it on this problem :

--------------------------------------------------------
MINOS 5.5: ignoring integrality of 2 variables
MINOS 5.5: infeasible problem (or bad starting guess).
0 iterations
Nonlin evals: obj = 7, grad = 6, constrs = 7, Jac = 6.
x0 = 1
y0 = 1
x1 = 1
y1 = 1
x2 = 1
y2 = 1
--------------------------------------------------------



And this is what I get when I add "option solver bonmin" in the problem
file :

-------------------------------------------------------------------------------------
 bonmin:

******************************************************************************
This program contains Ipopt, a library for large-scale nonlinear
optimization.
 Ipopt is released as open source code under the Common Public License (CPL).
         For more information visit http://projects.coin-or.org/Ipopt
******************************************************************************

IpOp0008I
              Num      Status      Obj             It       time
IpOp0009I     1        FAILED      -0             3000     2.81218
IpOp0010I     r1       FAILED      -0             3000     2.78817
Ipopt exited with error code -1 Maximum Iterations Exceeded
can't open /tmp/at23241.sol
-------------------------------------------------------------------------------------

What am I doing wrong ? Where to start debugging ? :)




>From Nantes, France,
BERGER Nicolas